Electric vehicles, plug-in hybrid electric vehicles and the like are known as electrically driven vehicles, which are designed to feed an electric machine for the drive of the vehicle from a primary energy store, for example a traction battery or a traction accumulator. Furthermore, vehicles of this type usually also have an additional energy store, for example a lead battery, which supplies electrical energy to low-voltage consumers in the vehicle, for example various electronic control devices and consumers, which provide comfort and convenience functions, and the like.
Vehicle lead batteries are designed to be charged during vehicle operation and to supply electrical consumers with energy while the vehicle is stopped or parked. When lead batteries of this type are also charged continuously, while the vehicle is stopped, this leads to a high water loss of the lead battery, which may lead to a premature failure of the lead battery, if the water loss is not counterbalanced. In addition, long service lives may promote corrosion of the battery plates and/or a development of internal battery short circuits.
Modern electric vehicles or plug-in hybrid electric vehicles usually have a DC voltage of approximately 12V for the low-voltage energy supply of the vehicle. This low voltage in vehicles of this type may be provided by a lead battery as an additional energy store, which is electrically connected to a DC voltage transformer (DC to DC converter) or a rectifier (AC to DC converter). Since the low-voltage energy supply is required for the operation of the different electronic control devices during the charging of the primary energy store or the traction battery of the vehicle, a charging voltage is applied continuously at the lead battery. The charging periods may exceed seven hours daily, and the lead battery is consequently exposed to much longer daily charging times, which may lead to a premature operating failure on account of the water loss, corrosion, or internal battery short-circuits. The situation is also exacerbated by the fact that the traction battery can be charged, for example at a charging socket, generally in an unattended manner and in a closed garage, and by the fact that lead batteries of decreasing function tend to produce hydrogen gas in greater quantities than usual and heat up during charging.
Various types of electric vehicle charging devices, which can charge a primary energy store installed in a vehicle by means of an AC voltage source arranged externally of the vehicle are known. In one such device, an AC to DC converter converts the AC voltage externally of the vehicle into a DC voltage and provides this to consumers in the vehicle while the primary energy store is charged by the external AC voltage source. An auxiliary energy store is also provided that can supply the consumers with electrical energy. While the primary energy store is charged by the external voltage source, a diode connected in series with the auxiliary energy store prevents a charging of the auxiliary energy store to increase the charging efficiency when charging the primary energy store. However, the auxiliary energy store can be discharged to supply the consumers in the vehicle. During the charging of the primary energy store, however, the discharge of the additional energy store is prevented in that a control device controls the output voltage of the AC to DC converter in such a way that the diode is operated in a blocking state.
In another known electrical circuit arrangement of a hybrid electric vehicle, a low-voltage battery in a low-voltage supply network of the vehicle can be separated from a high-voltage supply network including a high-voltage battery for driving the vehicle by an associated switch. The switch may be closed to charge the high-voltage battery using the low-voltage battery. Similarly, a low-voltage battery may be charged from a high-voltage battery.
Representative strategies for charging a low-voltage battery in an electric or hybrid electric vehicle are disclosed in U.S. Pat. No. 8,186,466 B2; US 2007/0210743 A1; and US 2012/0169281 A1, for example.